The present invention relates to an apparatus that can accurately detect misfires in a multi-cylinder internal-combustion engine, and more particularly, to an apparatus that can accurately count the number of misfires for each cylinder even if a vehicle equipped with the engine runs on rough roads and even if the vehicle body experiences jolting (which can actually occur as a result of a misfire or from some other cause).
A conventional combustion status detector used in an internal-combustion engine can be equipped with a revolving speed detection device for detecting the revolving speeds of the internal-combustion engine for each of the required revolving angle periods of its crankshaft that are shorter than the ignition period of the engine. An averaging device can also be provided for averaging the detected engine speeds by a period equivalent to one revolution of the crankshaft, thereby permitting detection of the combustion status of the engine. For example, the speed can be averaged for one minute to determine the combustion status of the engine, which, in turn, can be used to determine the combustion condition. In conjunction with this, a misfire detection capability can be realized based on the combustion status detector. The misfire detection device can include a deviation calculation device for calculating the deviation between the average value engine speed calculated by the averaging means and the average value calculated during the time from the calculation of the calculated average engine speed to the required ignition period. A deviation averaging device is also included for averaging the foregoing deviation on the basis of a period equivalent to the foregoing ignition period. Also, a combustion status detection device is provided for detecting the combustion status of the engine on the basis of the foregoing averaged deviation. Such an arrangement is described, for example, on Page 2 of Japanese Application Patent Laid-Open Publication No. Hei 07-119536 (hereinafter referred to as Reference 1)
In Reference 1 noted above, a sufficiently accurate misfire detector has been achievable under ordinary driving conditions because the filter used in this application can extract primarily the frequency components of the misfire and effectively cut off the components of adjacent higher and lower frequencies. In the arrangement discussed in Reference 1, a four-cylinder internal-combustion engine is dealt with as an example. However, if the number of internal-combustion engine cylinders is larger than four, the revolving angle of the crankshaft for one explosion stroke becomes smaller. This can lead to a deterioration in the detection accuracy of the time required for one explosion stroke. As a result of this deterioration, it is not possible to realize a filter with sufficient frequency characteristics to accurately realize detection of a misfire. The accurate detection of a misfire becomes even more of a problem when the vehicle travels over rough roads, as will be discussed below.
Typically, in a revolving speed detector such as discussed above, a magnetic sensor changes its output according to the distance between the sensor and the teeth of the ring gear. This change in output can be plotted as a wave form having recurring peaks, each peak being indicative of one explosion stroke, as shown in FIG. 1(b). A time period T between the recurring peaks indicates the time required for one explosion stroke, and this time period can be monitored to determine the combustion status.
Specifically, when no misfire occurs, the time period T is uniform. However, when a misfire occurs, the time period T changes because the misfire decreases the rotation speed, thereby the time interval T increases. Normally, this change in time period can be detected by a filter which indicates a misfire. However, if a vehicle is traveling on a rough road, or if the vehicle experiences jolting for some other reason, including an after effect of a misfire, the period T changes similarly to that created by a misfire. Accordingly, prior art systems often inaccurately diagnosed a misfire even when no such misfire existed. This problem becomes even more likely in the case of a six or more cylinder engine (as opposed to a four cylinder engine) because of the deterioration of the revolution detector accuracy, as noted above.
An object of the present invention is to realize a misfire detector that accurately detects misfires even in cases where the vehicle is driven over a rough road surface and or in situation involving noise such as jolts of the vehicle body, by using two filters to extract the intended frequency components more effectively.